Memory devices are typically provided as internal, semiconductor, integrated circuit devices in computers or other electronic devices. There are many different types of memory including random-access memory (RAM), read only memory (ROM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), and flash memory.
Flash memory has developed into a popular source of non-volatile memory for a wide range of electronic applications. Flash memory typically uses a one-transistor memory cell that allows for high memory densities, high reliability, and low power consumption. Changes in threshold voltage of the memory cells, through programming (which is often referred to as writing) of charge storage structures (e.g., floating gates or charge traps) or other physical phenomena (e.g., phase change or polarization), determine the data value of each cell. Common uses for flash memory include personal computers, personal digital assistants (PDAs), digital cameras, digital media players, cellular telephones, solid state drives and removable memory modules, and the uses are growing.
Electronic systems, such as memory systems, often include one or more types of memory and that memory is typically coupled to one or more communications buses within the memory system. Time varying signals in such systems are often utilized to transfer information (e.g., data) over one or more conductors often referred to as signal lines. These signal lines are often bundled together to form a communications channel, such as an address bus or a data bus, for example.
To meet the demands for higher performance operating characteristics, designers continue to strive for increasing operating speeds to transfer data across these communications buses within these systems. However, one issue with increased data transfer rates is maintaining signal integrity during these bursts of data on the various bus signal lines of the memory system. As these transfer rates increase, the impedance characteristics of a data bus become more pronounced. Capacitive and inductive characteristics of the signal lines may begin to distort the signal waveforms on the data bus at these higher data rates. Waveforms may begin to spread out and/or reflections may occur at locations of unmatched impedance on the data bus signal lines, for example. Signal integrity (e.g., data integrity) can be affected when an impedance (e.g., output impedance) of one or more nodes of a memory device coupled to a communications bus is not properly matched to the impedance of the communications bus. Impedance mismatch might result from process variations, temperature variations and voltage (e.g., power supply potential) variations in a memory device, for example. Thus, it is typically desirable to reduce these effects in order to reduce the likelihood of data corruption as data is transmitted on a data bus, for example.
Reducing the effects of impedance mismatch often involves the inclusion of adjustable termination devices within signal driver circuits, e.g., output driver circuits. These termination devices, often referred to as pull-up legs and pull-down legs, generally involve switchable resistances between a signal line and a voltage node, i.e., each termination device of a signal driver circuit can selectively connect the signal line to a voltage node through a resistance. For example, pull-up legs generally involve switchable resistances between a signal line and a supply voltage node, e.g., Vcc, while pull-down legs generally involve switchable resistances between the signal line and a reference voltage node, e.g., Vss.
A signal driver circuit may include multiple termination devices, and the resistance of each termination device is often adjustable. For example, the resistance may take the form of multiple resistors selectively connected in parallel between the signal line and the voltage node. By altering which resistors are connected, e.g., through the use of trim settings, the resistance of the termination device can be adjusted.
The resistance of a termination device is often calibrated to match (or closely match) a reference resistance in order to achieve a desired impedance. Because termination devices of a signal driver circuit often use the same configuration of resistors, and because process variation between devices during fabrication would be expected to be minimal within a signal driver circuit, settings determined for one termination device of a signal driver circuit are often copied to other termination devices. However, situations may arise where a desired impedance may not be achieved when adjusted in this manner.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for alternative apparatus for, and methods for, adjusting impedance in a signal driver circuit.